Most modern radio transmitters must precisely control their radiated radio frequency (RF) output power to within a certain predetermined limit. This requirement is especially important in the design of cellular radio telephone and similar transmitters which must provide several output power levels within certain tolerances over a wide range of operating temperatures.
A typical controlled-output amplifier circuit known in the prior art uses a controllable-gain amplifier stage arranged in a feedback loop. The output power level of the amplifier stage is determined typically by using a detector diode and a capacitor to form a half wave rectifier. The detector output signal is then compared to a reference voltage to develop an error signal which is fed back to control the gain of the amplifier stage, and thus to maintain the output power level.
Unfortunately, most present-day detector diodes possess characteristics which may produce adverse operational effects in certain applications. For example, the forward voltage drop of a detector diode is highly temperature-dependent; it usually varies on the order of several milli-volts per degree centigrade (mV/.degree.C.). If the application calls for a relatively low output power level (on the order of 100 milliwatts (mW) or less), then the temperature dependence of the forward voltage drop of the diode will be fairly large in comparison. Thus, the variations in forward voltage drop with temperature over a range of operating temperatures will often introduce significant errors in the detector output signal.
A conventional solution to minimizing the effect of temperature drift in the forward voltage drop of the detector diode is to use some form of temperature compensation. In one embodiment of this technique, such as that disclosed in Walczak, et al. U.S. Pat. No. 4,523,155, on June 11, 1985, and assigned to Motorola, Incorporated, a second, compensation diode is arranged to bias the detector diode at a conduction threshold. If the detector diode and compensation diode have "matched" temperature characteristics, their forward voltage drops track one another as the temperature varies, and thus the detector diode remains biased at a constant conduction threshold voltage which remains constant with temperature.
A second problem arises due to the fact that the peak voltage of the RF signal output by the amplifier stage must be limited to a narrow range in order to prevent the diode from being reverse-biased or driven into saturation. Thus, the amplifier circuit must be operated over a limited dynamic range. U.S. Pat. No. 4,602,218, issued to Vilmur, et al. on July 22, 1986, and assigned to Motorola, Incorporated, discloses one technique for addressing this problem, wherein a logarithmic or some other type of non-linear amplifier is inserted between the amplifier stage and the detector diode. The non-linear amplifier substantially compresses the dynamic range of the amplifier output signal, which, in turn, has the effect of increasing the range of amplifier output voltages over which the detector diode exhibits acceptable characteristics.
Yet another solution to both of the aforementioned problems is disclosed in U.S. Pat. No. 4,760,347, issued to Li. et al. on July 26, 1988, and assigned to NovAtel Communications, Ltd., the assignee of the present application. In that technique, the detector diode may be biased to any one of a number of pre-selected bias states in accordance with changes in the desired output power level. This technique has the dual advantage of eliminating the need for a second temperature compensating diode as well as increasing the available dynamic range.
While these techniques work for their intended purpose, they do not solve the aforementioned problems entirely. For example, the temperature compensation diode approach requires a set of expensive matched diodes. Similarly, the non-linear amplifier approach requires matched components such as a differential transistor pair. And the bias control approach requires a fairly complicated circuit to change the detector bias to a number of different levels in accordance with the desired output power level.